For example, in an infrared microscope, infrared light or visible light can be selectively irradiated to a sample. Since common optical components are used in at least a part of an optical system for irradiating the infrared light to the sample and an optical system for irradiating the visible light to the sample, the number of components is decreased (for example, see Patent Document 1 below).
It is considered to use an objective lens (transmission element) used in a biological microscope as part of the optical system. However, since the objective lens is normally corrected for aberration in a visible region, there is a problem that the influence of chromatic aberration increases and the imaging performance deteriorates when the objective lens is used in an infrared region. Also, since absorption occurs in the infrared region due to a glass material of the lens, the transmittance is greatly decreased and the usable wavelength range is extremely limited.
For such a reason, in a general infrared microscope, a reflection optical system in which light reflected by an optical member is irradiated to a sample is used instead of a transmission optical system in which light transmitted through an optical member is irradiated to a sample. In the case of using such a reflection optical system, a problem of chromatic aberration does not arise differently from the case of using the transmission optical system. Further, since the reflectance of not only the visible region but also the infrared region is high in an aluminum vapor deposition mirror usually used in the reflection optical system, it is possible to use the infrared microscope in a wide wavelength region from the visible region to the infrared region.
As optical components included in the reflection optical system, for example, a Cassegrain reflector including a primary mirror and a secondary mirror which are coaxially disposed is used. The Cassegrain reflector is a so-called Schwarzschild reflection objective mirror and has an optical arrangement which is very similar to a Cassegrain astronomical telescope.
Further, in the infrared microscope capable of performing the transmission measurement, Cassegrain reflectors having the same specifications (magnification, numeral aperture NA, etc.) as imaging objective mirrors are often used in combination as condenser mirrors for collecting infrared light and irradiating the light to a sample. In this case, for example, a pair of Cassegrain reflectors are disposed above and below the sample. The pair of Cassegrain reflectors includes a Cassegrain reflector (a lower Cassegrain reflector) which focuses the infrared light to the sample from below and a Cassegrain reflector (an upper Cassegrain reflector) which images transmitted light directed upward from the sample.
FIG. 1 is a schematic cross-sectional view illustrating an example of an internal configuration of a Cassegrain reflector 200. Further, FIG. 2 is a schematic plan view illustrating an example of an external configuration of the Cassegrain reflector 200. FIG. 2 is a diagram illustrating the Cassegrain reflector 200 when viewed from the direction of an arrow A in FIG. 1.
The Cassegrain reflector 200 is provided with, for example, a primary mirror 201 and a secondary mirror 202. The primary mirror 201 includes a reflection surface 211 formed as a spherical concave surface. Meanwhile, the secondary mirror 202 includes a reflection surface 221 formed as a spherical convex surface. The reflection surface 221 of the secondary mirror 202 has a diameter smaller than the reflection surface 211 of the primary mirror 201.
The primary mirror 201 and the secondary mirror 202 are retained by a hollow casing 203. The primary mirror 201 and the secondary mirror 202 are attached to the casing 203 so that the centers of the reflection surfaces 211 and 221 are located on the same axial line L. More specifically, the reflection surface 221 of the secondary mirror 202 faces the reflection surface 211 of the primary mirror 201 while being separated therefrom.
A surface facing the reflection surface 211 of the primary mirror 201 in the casing 203 is provided with, for example, a circular opening 231. As illustrated in FIG. 2, the secondary mirror 202 is supported by a plurality of supporting rods 232, radially extending from the circumferential edge of the opening 231 toward the axial line L, from the lateral side (the outside in the radial direction) to be located at the center portion of the opening 231. Accordingly, the opening 231 is defined into a plurality of regions by the supporting rods 232 at the side of the secondary mirror 202.
As illustrated in FIG. 1, the primary mirror 201 is provided with an opening 212 on the axial line L. The infrared light or the visible light is incident from the opening 212, is reflected by the reflection surface 221 of the secondary mirror 202, and is reflected by the reflection surface 211 of the primary mirror 201. At this time, a part of the light reflected by the reflection surface 211 of the primary mirror 201 is shielded by the secondary mirror 202, but the other light is emitted from the side of the secondary mirror 202 through the opening 231. The light emitted from the opening 231 is focused to a measurement position P at the outside of the casing 203.